Field of the Invention
The present invention relates to a radiation imaging apparatus and a radiation imaging system.
Description of the Related Art
In medical and other fields, there have been put into practical use radiation imaging apparatuses, which are configured to receive irradiation of a radiation ray or the like after being transmitted through a subject, hold image signals of the subject's image, and output electric charge signals that reflect the held radiographic image. As an example of the radiation imaging apparatuses, there is given one that is configured to combine a photo sensor with a semiconductor material or a fluorescent substance that generates electric charge in an amount determined by the amount of radiation irradiating the apparatus. This radiation imaging apparatus has an area sensor unit, which includes thin film transistor (TFT) switches and photoelectric conversion elements. The TFT switches are formed mainly of amorphous silicon on a glass substrate in a two-dimensional pattern. The photoelectric conversion elements convert a radiation ray into electric charge. The radiation imaging apparatus switches the operation of the area sensor unit in synchronization with a control signal, which is input via an I/O device connected to the radiation imaging apparatus.
Introducing this type of radiation imaging apparatus requires, unlike film-type radiation imaging apparatuses, which are older technology, connecting the radiation imaging apparatus via an I/O device to a radiation generating apparatus and a control system that are located in a hospital. The I/O device varies from maker to maker, which means that an alteration to equipment to be connected to the I/O device and, in some cases, even replacement of the whole radiation generating apparatus, are necessary. The introduction cost is therefore problematically high.
In the radiation imaging apparatus, the gain of a column amplifier is set based on a predicted imaging dose range, the pixel sensitivity, and the noise of a readout circuit. However, a preset amplification ratio may not work depending on the performance of the radiation imaging apparatus and the positional relation between a subject and the radiation imaging apparatus. Specifically, there are cases where the positional relation of a subject with the radiation imaging apparatus makes the dose of the incident radiation ray unexpectedly large and where a radiation ray transmitted through a thick subject falls farther short of the radiation imaging apparatus than expected. In such cases, the excessively large dose or the excessively small dose can degrade image quality in an area that needs diagnosis most (an interested area).
Accordingly, using the radiation imaging apparatus involves such work as modifying the length of radiation irradiation time empirically by taking radiation conditions and the positional relation with the subject into consideration, and listing several hundred combinations of radiation conditions to register the conditions in advance, which are heavy burden to a user of the radiation imaging apparatus.
A radiation detecting apparatus described in Japanese Patent Application Laid-Open No. 2004-85383 is provided with pixels for detecting the X-ray irradiation amount aside from pixels for forming an image. X-ray irradiation can be detected by monitoring the dose of an X-ray that irradiates a sensor with the use of these detecting pixels. This technology eliminates the need for a work of connecting an I/O device to an X-ray generating apparatus when the radiation detecting apparatus is introduced to a hospital, thereby reducing the introduction cost. The radiation detecting apparatus capable of constant monitoring of the X-ray dose can also be applied to automatic exposure control (AEC) of the X-ray irradiation amount in which a session of X-ray irradiation is monitored to stop X-ray irradiation when the right X-ray dose is reached.
However, the radiation detecting apparatus of Japanese Patent Application Laid-Open No. 2004-85383, where signals of the X-ray irradiation amount detecting pixels are read out by a shared readout circuit, which reads out signals of the image forming pixels as well, has the following problems:
The first problem is power consumption. When performing a readout operation, the readout circuit consumes a very large amount of current in order to reduce the noise of the readout circuit itself. Detecting X-ray irradiation involves constant readout of the X-ray irradiation amount detecting pixels, which means that a huge amount of power is consumed to obtain one X-ray image. An increase in power consumption gives rise to problems such as limitations put on the length of drive time by heat generation of equipment, and a shortened drive time in a battery-driven radiation imaging apparatus.
The second problem is the time required to output data. The readout circuit, which is designed so as to be capable of reading out a plurality of pixel columns, has a multiplexer therein to select signals sent from pixel columns sequentially and transfer the selected signals to the outside of the readout circuit. The radiation detecting apparatus of Japanese Patent Application Laid-Open No. 2004-85383 takes time to estimate the X-ray dose because the readout of the X-ray irradiation amount detecting pixels for the purpose of X-ray dose detection is accompanied by the readout of the other pixels, i.e., the image forming pixels. In the case of the short irradiation of an intense X-ray, lengthy readout of the detecting pixels means that the right X-ray dose has been exceeded by the time a control circuit determines that there has been X-ray irradiation, and therefore hinders proper control by AEC.